The invention relates to a method of operating a gas engine, i.e. a piston engine running on gaseous fuel.
Typically, the gaseous fuel used as main fuel in a gas engine is fed into the combustion chamber of the engine mixed with air. It is ignited by injecting a very small amount of ignition fuel, which is ignited due to the conditions existing in the combustion chamber and thereby also ignites the gaseous fuel. In this case, as far as the gaseous fuel is concerned, the engine is run according to the Otto process. Conventionally, the ignition fuel is liquid fuel and is injected using a common rail fuel injection system. The common rail fuel injection system is advantageous because it allows the combustion process to be controlled so that the exhaust gas composition is favorable with respect to emission concentrations. However, the common rail system is subject to disadvantage because it may malfunction, leading to an unexpected shutdown of the engine.
In the event that a gas engine is used as a marine engine, the fuel feed system must meet special requirements in order to assure safety and reliability. The most important such requirement is that unexpected shut down of the engine can not be allowed under any circumstances. Because of this, a gas engine used as a marine engine may be provided with a backup fuel feed system based on using only liquid fuel. Typically the backup fuel feed system is hydromechanically controlled, because hydromechanical fuel feed systems are highly reliable. In case there is a malfunction in the common rail fuel feed system affecting running of the engine, the engine transfers from running on gaseous fuel as the main fuel and using liquid ignition fuel supplied by the common rail system to running only on liquid fuel supplied by a hydromechanical fuel feed system and the engine is then run according to the Diesel process.
The backup use of liquid fuel is, however, problematic in the sense that with a hydromechanical system it is impossible to control the combustion process so that it will be the cleanest possible, since the combustion products include, for example, uncombusted hydrocarbons and nitrogen oxides in substantially larger amounts than in normal gas operation. Further, with backup operation, as described above, all the power is produced by using liquid fuel, whereby the fuel consumption is multifold compared to the consumption of liquid fuel as ignition fuel in gas operation. Consequently, the tanks for the liquid fuel will have to be dimensioned taking the space requirement of the marine vessel into consideration, which affects the load capacity of the vessel. Thus, the possibility exists of having insufficient liquid fuel available for use in backup or reserve fuel operation.
The object of the invention is to provide a method by means of which the problems relating to the prior art can be minimized. An especial object of the invention is to provide a method by means of which, among others, the operational reliability of the gas engine can be guaranteed in all situations and at the same time minimize the risk of running out of liquid fuel and to provide a method by means of which the exhaust emissions can be reduced even in backup operation conditions of the gas engine.